Fallback to service discovery if caps not supported
It seems that Gajim is the only client supporting caps properly so far.
As we now use caps to check for features, this brings a few problems. All clients seem to support service discovery and announce for example support for file transfers. But as we only check caps, we can't send to those users. We need to do one service discovery when a user doesn't support caps and cache that. That way, we would be compatible with other clients for file transfers / MUC again.
Asterix, I guess this is a job for you :þ.